1. Field of the Invention
This invention relates to engine exhaust gas systems for internal combustion engines, and more particularly relates to increasing exhaust flow temperatures for aftertreatment devices.
2. Description of the Related Art
Environmental concerns motivate emissions requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which all engines must comply. Generally, emission requirements vary according to engine type and intended use. Emission requirements for compression-ignition (diesel) engines typically dictate the release of diesel particulate matter (DPM), nitrogen oxides (NOX), and unburned hydrocarbons (UHC).
The need to comply with emissions requirements encourages the development of exhaust gas aftertreatment systems. Aftertreatment systems frequently include components requiring periodic temperature increases. A diesel oxidation catalyst (DOC) aftertreatment device may require an exhaust flow temperature increase to oxidize UHC in the exhaust stream. A NOX adsorption catalyst (NAC) aftertreatment device may require an exhaust flow temperature increase to facilitate a regeneration event within the device. A N diesel particulate filter (DPF) aftertreatment device may require exhaust flow temperature increases to oxidize collected soot particles. Other aftertreatment devices such as 3-way catalysts and 4-way catalysts may simultaneously oxide and reduce exhaust gas components, and require regeneration. Temperature increases for multi-purpose aftertreatment devices may require narrow exhaust flow temperature range targets to meet competing temperature requirements.
Currently, several methods for increasing exhaust flow temperature exist. One method comprises “dosing” the exhaust flow with fuel such that a device in the aftertreatment system may burn the fuel to facilitate an increase of the exhaust flow temperature. Often, dosing is introduced “in-cylinder” to ensure that the fuel is thoroughly mixed in the exhaust flow by the time the fuel reaches the aftertreatment system. One challenge of dosing systems is a significant time lag between the beginning of dosing and the increase of temperature within the aftertreatment system. A major challenge experienced with in-cylinder dosing is that, to prevent fouling of components ion the recycle stream, and unknown quantities of fuel recycling to the intake air and disrupting the designed engine torque and air-fuel ratios, the exhaust gas recirculation (EGR) system is usually closed during periods of dosing. While dosing may facilitate temperature increases in the exhaust flow and thereby regenerate the aftertreatment components, emissions may spike during this dosing phase with the EGR shut off.
Another method for increasing exhaust flow temperature comprises manipulating the timing of introduced fuel to the combustion chamber. Typically, this is accomplished by retarding the entry of fuel. However, altering the timing of introduced fuel has limited theoretical effectiveness for increasing exhaust flow temperatures. One further method known in the art utilizes modest changes in exhaust valve timing to produce pressure pulses across an aftertreatment device. Exhaust valve timing changes in the present art are modal, responding in an on-off fashion with no control over generated pressure pulses. Further, the pressure pulses produced by this method have only limited affect on temperature generation within the aftertreatment device.
Current exhaust temperature increase methods especially lack capability at low engine exhaust flow and power output. The responses to engines that operate at low power for extended periods in the current art include lighting a lamp to indicate that user intervention is necessary, and engaging an invasive mode to purposefully regenerate the aftertreatment system—for example requiring the user to stop a vehicle and engage a specific regeneration mode.
Current methods of increasing exhaust flow temperatures to regenerate aftertreatment components have limited effectiveness, or significantly increase emissions during operation. Other challenges of current methods include slow response to an exhaust temperature change request, insufficient temperature generation in the exhaust flow, and insufficiently precise temperature ranges produced for effective and efficient operation of aftertreatment components.